Well, first, where do you live? In some places it would be illegal to move her before she is 40 lbs. After that, I think it depends more on personal comfort level- some people are comfortable boostering a 5 year old before they reach 40 lbs and other people aren't. I'm not sure we have any conclusive proof that one is safer than the other for a child that age IF they can sit properly in a booster. I don't think many people here would endorse moving a still 4 year old to a booster full time, though.
It would also be helpful if you'd post what seats your kids are in now along with approximate weights/heights (preferably torso heights). That way we can have a clearer picture and provide more specific suggestions for your situation :thumbsup: .
Also, there is no reason a 1 year old must move out of the infant seat. Many infant seats will accommodate children past their 1st birthday. What is important is if he still fits the seat. He would need at least 1 inch of hard shell above his head and be under the weight limit for your seat. Since, to be safest, he will need to rear face for at least another year (preferably to the limits of his convertible seat) it doesn't really matter if it's a properly used infant seat or a properly used convertible seat, when it comes to safety